1. Field of the Invention
The present invention relates to the art of welding and is specifically concerned with methods for electroslag welding with consumable plate electrodes.
The invention may find the most successful application in heavy and metallurgical machine building for manufacturing large-size and heavy blanks such as for rotors of large alternators, for supporting rolls of unique sheet mills, for trunnion plates of converters, etc.
2. Description of the Prior Art
Methods for electroslag welding of large-thickness blanks with electrodes either movable or stationary with respect to the end faces being welded together, wherein adding materials are fed onto the slag bath surface in the course of welding, are well known in the art.
One of electroslag welding methods involves feeding the adding material with the aid of a metallic or refractory tube which extends into the molten metal bath through the slag.
Such a method of feeding the adding material fails to provide a uniform distribution thereof along the weld axis. Strong electromagnetic fields between the electrodes and the walls of the workpiece being welded promote sticking of the adding material to the feeding tube, which eventually results in clogging the tube and thus terminating the feed of the material in the course of welding.
Feeding a large amount of the adding material on a relatively small portion of the welding gap upsets the thermal balance of electroslag welding process and brings about a chemical inhomogeneity of the weld metal over the depth thereof and hence also a nonuniformity of the physico-mechanical properties of the metal.
There is also known a method for electroslag welding with consumable plate electrodes ("Elektroshlakovye pechi" Ed. by B. E. Paton and B. I. Medovar. Kiev, "Naukova dumka", 1976, pp. 226-228).
When carrying out the conventional method of welding, the work-pieces to be welded are arranged on a bottom plate and the end faces thereof are set at the predetermined angle of divergence.
In welding by this method, use is taken of two pairs of consumable electrodes: one pair of movable electrodes fed into the welding gap with the aid of an electrode holder adapted to move on a vertical guiding coloumn, and one pair of stationary electrodes installed with the aid of special brackets directly on blanks of workpieces to be welded. All the consumable electrodes are installed in one row along the edges to be welded.
The electrodes being installed, voltage is applied from a power transformer, whereupon a slag bath is established with the use of a siphon device. When the predetermined welding current is attained, a mechanism for vertically moving the movable consumable electrodes is switched on, and a round metallic adding material is fed into the spacing between the electrodes.
When the welding process is completed, the mechanism for moving the movable electrodes is switched off, feeding the ground adding material is stopped, the power transformer is switched off, the siphon device is removed, and the stubs of the electrodes are taken off.
With this method, however, the weld shrinkage strains cause the welding gap to open, i.e. the workpieces being welded together to turn relative to a vertical axis towards increasing the gap, at the initial moment of welding and to close in the process of welding.
Such a turning of the workpieces in one or other direction results in shifting the consumable electrodes from their initial position and hence in a random variation of the gaps between the electrodes and the workpieces under welding; this in its turn gives rise to an unsymmetrical nonuniform fusion of the edges and sometimes even to a lack of fusion of one of the edges over the weld height, and thus greatly degrades the quality of the weld joint.
Studies have revealed that variations in the distance between the electrodes and the surfaces being welded together also destabilize the welding process.
Moreover, welding together workpieces by this prior art method lowers the crystallization-crack resistance of the built-up metal because of the nonuniformity of fusion of the edges and consequent formation of a weld whose shape factor varies over its thickness.